Light energy, the frequency of which is ν, is given as an integer multiple of hν (where h is Planck's constant). When the value of a certain physical quantity is given as an integer multiple of a basic quantity (unit quantity), the basic quantity is called a quantum. That is, hν is a quantum. Furthermore, a quantum in the optical energy range is particularly called a photon.
Recently, a cryptography method using a quantum phenomenon started to be researched. The reason for this is that conventional cryptographic systems are established using mathematical methods, but, with the development of quantum computers, conventional cryptographic systems based on mathematical methods are starting to be exposed to risk, and thus a cryptographic system using the quantum phenomena of physics can be the alternative to them.
FIG. 1 is a diagram illustrating a method of determining whether eavesdropping has occurred in a quantum cryptography scheme. In a quantum cryptographic transmission system, a cryptogram is transmitted in a way that makes it impossible to eavesdrop on the cryptogram.
The view a) of FIG. 1 denotes a transmitter and the b) of FIG. 1 denotes a receiver. The first row of a) of FIG. 1 is a plain text message which the transmitter desires to transmit, and the second row signifies polarization directions. That is, symbol × represents diagonal polarization and symbol + represents vertical/horizontal polarization. There is a 50-50 chance that a photon polarized in a diagonal direction will pass through a polarizing plate polarized in the horizontal direction or the vertical direction.
First, the transmitter and receiver ensure that information bit 0 is represented as vertical or /-diagonal polarization and information bit 1 is represented as horizontal or \-diagonal polarization.
The transmitter selects diagonal polarization or vertical/horizontal polarization to be used for each bit of a plain text message. As illustrated in the third row of a) of FIG. 1, the polarization direction of the first bit is selected as the diagonal direction and that of the second bit is selected as the vertical/horizontal direction. Then, the first bit 1 is encrypted in the \-directional polarization, and the second bit 1 is encrypted in the horizontal polarization.
The first row of the view b) of FIG. 1 represents signals received by the receiver. The signals received by the receiver are identical to those of the third row of the view a) of FIG. 1, and the second row of b) of FIG. 1 represents the polarization directions of the polarizing plate selected by the receiver to decrypt signals received from the transmitter. The third row of the view b) of FIG. 1 represents polarization directions measured through the polarizing plate selected by the receiver, that is, a decrypted plain text message.
The receiver randomly selects the direction of the polarizing plate to be used for the decryption of encrypted signals. As a result, the results of decryption may be correct or incorrect. When the receiver selects polarization directions identical to those of the transmitter, the bits of a plain text message are recovered without error. For example, the first bit of cipher text transmitted by the transmitter is \-directional polarization. In this case, if the receiver selects diagonal polarization and then a \-directional polarizing plate, the photon transmitted by the transmitter is detected with 100 percent certainty. If the receiver selects a diagonal direction and then the /-direction, the photon transmitted by the transmitter is not detected with 100 percent certainty. As a result, in either case, the receiver can be sure that the polarization direction of light that it has received is the \-direction.
However, as the example of b) of FIG. 1, the first bit of the cipher text transmitted by the transmitter is \-directional polarization (the first row of the first bit). If the receiver selects a vertical/horizontal direction (the second row of the first bit) and then uses a vertical polarizing plate or a horizontal polarizing plate, the probability that the polarization value set and transmitted by the transmitter will be acquired is ½. That is, the receiver may correctly or erroneously decrypt the polarization of a photon transmitted by the transmitter. In the example of b) of FIG. 1, incorrect results were obtained (the third row of the first bit).
The receiver randomly selects polarization directions as described above, and then decrypts the polarization values of the cipher text transmitted by the transmitter in its own way. Thereafter, the receiver selects several bits and notifies the transmitter of polarization directions used for the decryption of bits (bits represented in the second row of b of FIG. 1). Thereafter, the transmitter again notifies the receiver of bits among the bits which are transmitted from the receiver, which correspond to the directions of the selected polarizing plate. Through the above-described method, the transmitter and the receiver determine which bits must be delivered. In the example of the table, the second, third, fifth, tenth and twelfth bits are such bits.
Thereafter, the transmitter and the receiver compare these bits with each other. When eavesdropping has occurred, the polarization value of a photon polarized by the transmitter and transmitted to the receiver is changed. When the bits are all the same, it is certain that no eavesdropping has occurred. If there is a changed bit value, it is certain that eavesdropping has occurred between the transmission and reception of a cryptogram.
In such a quantum cryptography method, a photon is used as the means of delivering a quantum cryptogram. A single photon may be used, or a photon flux, which is a collection of photons, may be used.
When a third party attempting to eavesdrop on a cryptogram acquires a photon in the case in which a single photon is used as the means for delivering a quantum cryptograph, the photon which is being transmitted to the receiver immediately disappears, so that the receiver can immediately detect the violation by the third party. However, the single photon may appear or be changed due not only to an attack by a third party but for various reasons, such as light scattering, so that a cryptography method using a photon flux is generally used.
FIG. 2 is a quantum cryptography method using conventional technology. The conventional technology is described below with reference to the drawing.
In FIG. 2, a transmitter 10 sets vertical/horizontal polarization to 0 and π/2 for a photon flux emitted from a light source and diagonal polarization to −π/4 and π/4. When transmitting the photon flux to a receiver 20, the transmitter 10 randomly selects one state among four polarization states 0, π/2, −π/4 and π/4 for the polarization of the photon flux, and then transmits it. The receiver 20 measures the polarization of a received photon flux using a beam splitter 300 and a photo-detector 400.
When the transmission of the photon flux is finished, the transmitter 10 notifies the receiver 20 of the vertical/horizontal direction and the diagonal direction, which are polarization states of respective photon fluxes, rather than the polarization of each photon flux, and the receiver 20 notifies the transmitter 10 of values of the vertical/horizontal direction or the diagonal direction, which are polarization states that it has measured. Thereafter, the transmitter 10 and the receiver 20 take the same polarization states, and retrieve the values of the taken polarization states as information. That is, in the vertical/horizontal direction state, polarization 0 corresponds to information bit “0” and polarization π/2 corresponds to information bit “1”. In the diagonal direction state, polarization −π/4 corresponds to information bit “0” and polarization π/4 corresponds to information bit “1”.
FIG. 3 is a diagram illustrating the method by which a third party acquires information in the quantum cryptography method using the conventional technology. In the above-described conventional quantum cryptography method using a photon flux, there is a problem in that information is easily acquired by a third party. The method in which the third party acquires information is described with reference to FIG. 3. First, there are the transmitter 10, the receiver 20 and the third party 30, who attempts to read transmission and reception information moving between the transmitter 10 and the receiver 20. When the transmitter 10 rotates the photon flux of the light source 100 to one of the four polarization states through a polarization rotator 200 and then transmits it to the receiver 20, as described in the above-described quantum cryptography method, the third party 30 acquires some of the polarized photon flux transmitted by the transmitter 10 using beam splitting 500. In the conventional technology, a photon flux has only one of four polarization states, so that the third party can determine the polarization value of the photon flux, which is transmitted to the receiver 20 by the transmitter 10, from some of the polarized photon flux acquired through the beam splitter 300 and the photo-detector 400.
Furthermore, since the conventional technology uses one photon flux, information may leak. That is, when the third party impersonates the receiver when communicating with the transmitter and impersonates the transmitter when communicating with the receiver, the third party can acquire all of the information between the transmitter and the receiver.
As described above, the quantum cryptography method using the conventional technology has a problem in that it is vulnerable to outside attacks, such as a beam-splitting attack or an impersonation attack.